In the refining process, wood is fed in chip form into a narrow gap between stator and rotor of a refiner together with water. The cross section of the gap narrows from the center of the refiner towards the outer perimeter. The surfaces of the rotor and stator have bars that have edges for breaking the wood material into fibers. The chips are defibrated and fibrillated during their passage through the refiner. Refining can take place in one refiner or it can be continued in subsequent refiners.
The chips are fed into the center of a refiner and strike first the edges of a breaker bar and the chips are broken into pieces. Refining of these pieces begins when the pieces strike each other as well as the refiner rotor and stator edges. Centrifugal force drives this coarse wood and fiber mixture outward in radial direction the disc gap stator and rotor plates and the gap becomes smaller. The interaction between rotor, stator and fiber defibrates and fibrillates the fiber material to the final freeness level. The collisions between fiber and rotor bar edges and collisions between fibers, friction between fiber and segment surfaces and internal friction in the fiber phase consume a considerable amount of energy. This energy is transformed into heat, which increases the temperature of the water and fiber and thus evaporates the water into steam. This steam has strong influence on fiber flow in the disc gap. Depending on the pressure conditions before and after the refiner, some of the steam flows toward the chip feed as a flowback steam and some flows forward with the fiber flow. The steam flow is restricted due to narrow disc gap and, because of this, the pressure and the temperature in the disc gap can be noticeably higher than those in the refiner housing or feed. The heat in the refining process changes the rheological properties of wood and fiber and has an important influence on the final fiber quality.
Very often two serial refiners are used in a refining process. In such a process once refined raw fibers exit from the first refiner together with steam and exhaust velocity of the steam and chips is used for transporting the fibers to the second refiner. Since large amount of steam is generated in the first refiner, excess steam has to be removed from the flow before feeding the chips to the second refiner. Therefore, the outflow of the first refiner is led to a cyclone wherein excess steam is separated. The separated steam can be fed into the first refining stage. From the cyclone the defibrated fibers are fed into the second refiner, wherein it is further refined to final freeness. After the second refiner the mixture of fibres and steam is fed into a second cyclone for separating steam and fibers.
This kind of process requires large amount of energy for each ton of fibers produced. One factor that increases the amount of energy needed, is the generation of steam in the refiners. Steam is needed for transporting the refined fibers within the refiner between the stator and rotor or rotor to rotor especially from first refiner to the second refiner. In present processes large amount of steam is needed for transporting the fibers. This extra steam has to be removed from the fiber flow before it is fed into the second refiner. Steam or water is also needed for cooling the fibers that is heated due to friction between the refiner plates. If the consistency of the fibers is not right, the dwell time of the mass in the refiner may become longer and the mass may overheat and become dark. Longer residense times also change the refining result and quality of the fibers.
In conventional process steam is separated from fibers in between the refining stages. This steam is typically used in paper machine and therefore it requires very high pressure to first stage refiners housing and second stage refiners feed.